Vor 9960 hz harmonic generator

ABSTRACT

A controlled harmonic generation technique is applied to the 9960 Hz subcarrier signal related to VOR navigation by means separably applying a 30 Hz variable phase test signal and a 9,960 Hz subcarrier reference phase signal employed in the VOR navigation system to a harmonic generating means which acts on the 9,960 Hz subcarrier alone by generating predetermined levels of harmonics of the subcarrier with subsequent linear recombination of the controlled level harmonic components with a 9,960 Hz fundamental signal devoid of such harmonics and the 30 Hz variable phase signal. A selectively controlled composite modulation signal which simulates the modulation from actual VOR ground transmitting stations is thus provided.

Waited ta tes 9atent Hemme 1 Apr. 24, 1973 VOR 9960 HZ HARMONHC3,355,539 11/1967 Munch et a1 ..328/I7 x GENERATOR 3,495,158 2/1970Gamett et al... ....328/16 x 3,568,034 3/1971 Shenfeld ....328/16 X [75]Inventor: William R. Hemme, Fairmont,

Minn- Primary Examiner-John S. I-leyman [73] Assignee: Collins RadioCompany, Cedar Attorney-*Rlchard Anderson et Rapids, Iowa [57] ABSTRACT[22] Filed: Sept. 3, 1971 I A controlled harmonic generation techniqueIs applied [21] Appl.N0.: 7 1 to the 9960 Hz subcarrier signal relatedto VOR navigation by means separably applying a 30 Hz varia- 52 us. Cl...328/l6, 328/14, 328/18, Phase test slgnal and a' 92960 subcarler328/23 reference phase signal employed in the VCR naviga- 51 rm. Cl...H03h 21/02 system 3 gFmemtmg means which acts on the 9,960 Hzsubcarrler alone by generating [58] Field of Search ..328/14,l6,17,18,

328 84 1 11 1 l2 1 l9 1 20 1 2 2 2O predetermined levels of harmonics ofthe subcarrier l with subsequent linear recombination of the controlledlevel harmonic components with a 9,960 Hz [56] References C'tedfundamental signal devoid of such harmonics and the UNITED STATESPATENTS 30 Hz variable phase signal. A selectively controlled compositemodulation signal which simulates the 2,894,133 7/1959 Bolie ..328/l6 Xmodulation from actual VOR ground transmitting sta- 3,007,36l l 1/1961Wayne {ions is thus provided 3,125,729 3/1964 Stone et a1.... 3,245,0014/1966 Barber ..328/l4 X 6 Claims, 3 Drawing Figures I3 996OHz NBANDPASS F'LTER ZFSEC NOMINAL 996OHz V V H- suBcARRIER REF '1"Li" "L 1\1I 1| 1 v 22 o v INPUT 24 HIGH PosITIvE HIGH GAIN j SLOPE GAIN A LIMITER2/ DIFFERENTIATOR 23 LIMITER 25 36 LEvEL 3 LEvEL 38 LEVEL ADJ ADJ j ADJ2x 996OHz 3x 996OHz 4x 996OHz BANDPASS BANDPASS BANDPASS FILTER FILTERFILTER 29 0 I'ZI TWO-POLE 3OKHz ROLL-OFF 34 Low PAss FILTER 33 E L'NEARMODULATION 30m L MIxER/ OUTPUT VAR F? AMPLlFlER ,4 INPUT Patented April24, 1973 3,729,683

2 Sheets-Sheet 1 vOR sIGNAL GE ERATOR //2 996OH2 REF sIGNAL 3OHzVARS|GNAL ISERI?I%RRE GENERATOR MODULATION VROFR GENERATOR A5 vOR REcEIvERUNDER TEs I3 996OHz Q BANDPAss F'LTER 2,;sE0 NOMINAL 996OH2 -ik-SUBCARRIER W R i/ OL]L -v H U INPUT I r HIGH POSITIVE HIGH GAIN I SLOPEF GAIN A LIMITER 2/ DIFFERENTIATOR 23 LIMITER 25 36 LEvEL 37 LEvEL 3aLEvEL T ADJ ADJ ADJ 2x 996OHZ 27- 3x 996OH2 4x 996OH2 BANDPASS BANDPASSBANDPASS FILTER FILTER FILTER P29 g0 ks/ TWO-POLE 3OKHz ROLL-OFF /32 34Low PAss ILTER LINEAR I MODULATION Hz L r MIxER/ OUTPUT VAR/p JAMPLIFIER l4 v INPUT FEG.2

Patented April 2- 1, 1973 2 Sheets-Sheet 2 SnCbO 2075:3002

VOR 9960 HZ HARMONIC GENERATOR This invention relates generally tosignal generating devices and more particularly to an improved signalharmonic generator by means of which selectively controlled levels ofpredetermined harmonics of a base frequency may be generated.

In the specific embodiment to be exemplified, the present inventionresides in the provision of a 9,960 H2 harmonic generator for usage intest equipment for testing variable omnirange (VOR) receivers byproviding a means for presenting to the receiver a controlled amount ofdistortion simulating the modulation components from actual VOR groundtransmitting stations so that the test represents as closely as possiblethe various signal environments in which the receiver may be required tofunction.

Accordingly, a primary object of the present invention is the provisionof a harmonic generator by means of which an undistorted base frequencyand preselected levels of various harmonics of the base frequency may begenerated.

The present invention is featured in the application of controlledharmonic generation techniques to the 9,960 Hz subcarrier signal relatedto VOR navigation by means separably applying a 30 Hz variable phasetest signal and a 9,960 Hz subcarrier reference phase signal employed inthe VOR navigation system to a harmonic generating means the 'lattermeans acting on the 9,960 Hz subcarrier alone, selectively generatingpredetermined levels of harmonics of the subcarrier; and subsequentlyrecombining the controlled level harmonic components with the referencevariable phase signal to provide a selectively controlled compositemodulation signal simulating the modulation from actual VOR groundtransmitting stations.

The harmonic generator of the present invention is featured in theprovision of high gain limiting, positive slope differentiation, andmultiple harmonic bandpass filtering techniques, with individual leveladjusted harmonic filter outputs being linearly combined with a cleanbase or fundamental signal devoid of such harmonies to provide a VORmodulation signal with precisely controllable harmonic levels.

These and other features and objects of the present invention willbecome apparent upon reading the following description with reference tothe accompanying drawing in which;

FIG. 1 is a generalized block diagram of a VOR test arrangementemploying a harmonic generator in accordance with the present invention;

FlG. 2 is a functional block diagram ofa VOR 9,960 Hz harmonic generatorembodiment in accordance with the present invention; and

FIG. 3 is a functional schematic diagram ofa particular embodiment of a9,960 Hz harmonic generator in accordance with the present invention.

The Federal Aviation Administration has announced intent to begincommissioning VOR ground stations on 50 kHz channels in the near future.Present VOR ground stations operate on 100 kHz spaced channels. With theadvent of 50 kHz spaced channels, existing VOR receivers are known to bevulnerable to particular adjacent channel harmonic reception to generatefalse VOR bearing indications. Tests have indicated that performance ofVOR receivers to strong 50 kHz adjacent channel VOR signals is very mucha function of the 9,960 Hz subcarrier harmonic sidebands of the strongadjacent signal. That is, a VOR receiver may indicate a false bearingsignal, may receive an audible identification signal, and may fail toproperly flag when the receiver is mistuned by 50 kHz from a strong VORsignal and when there is not a VOR signal on the channel to which thereceiver is tuned. A particular monitoring arrangement by means of whichthis situation may be monitored and annunciated is described and claimedin my co-pending application, Ser. No. 61,594, entitled VOR AdjacentChannel Sensor filed Aug. 7, 1970, and assigned to the assignee of thepresent invention.

The harmonic generator of the present invention, as will herein bedescribed, provides a means for testing VOR receivers for theirsusceptibility to the above described and referenced adjacent channelVOR signals. More particularly, the generator to be described provides ameans for presenting to the receiver to be tested precisely controlledamounts of 9,960 Hz subcarrier harmonic distortion. As above discussed,the generator provides a controlled amount of distortion simulating themodulation from actual VOR ground stations so that the test mayrepresent as closely as possible the signal environment in which thereceiver is required to function. The maximum 9,960 Hz subcarrierharmonic levels that a VOR ground transmitting station is permitted toemanate are defined by ICAO Annex 10 as follows:

9960 H7. 0 db (REF) Second harmonic 30 db Third harmonic 50 db Fourthharmonic db The harmonic generator of the present invention provides ameans to simulate the'modulation from a VOR ground station and thereby aVOR receiver susceptibility to 9,960 Hz subcarrier harmonics from astrong adjacent channel signal can be tested.

With reference to FIG. 1, the VOR 9,960 Hz harmonic generator providedby the present-invention may be inserted between a VOR signal generator10 (such as a Collins type 4798-3) and a VOR RF generator 15 (such as aBoonton 211A) such that control harmonic generation of the 9,960 Hzsubcarrier can be accomplished. VOR signal generator 10 provides a 9,960Hz reference phase signal conventionally frequency modulated at a 30 Hzrate on output 11. A 30 Hz variable phase signal is provided on outputline 12 and each of the reference and variable phase signals 11 and 12are applied as inputs to the harmonic generator 13 of the presentinvention. The 30 Hz variable phase signal 12 and the 9,960 Hzsubcarrier reference phase signal 11 must be applied separately to theharmonic generator 13 such that the 9,960 Hz subcarrier can be acted onalone. The 9,960 Hz subcarrier, its harmonics, and the 30 Hz variablephase signal, as will be described, are recombined in the harmonicgenerator 13 to form a controllable composite modulation signal 14 forapplication to the VOR RF generator 15, the latter being employed tosupply the carrier frequency in any one of 50 kHz steps within the VORdefined frequency range of 108-1 18 MHz. The output 16 from the RFgenerator 15 thus simulates an actual received VOR signal from a groundstation for application to a VOR receiver 17 which may be under test.The harmonic generator 13 of the present invention as will be furtherdescribed provides a means for selectively and precisely controlling thelevels of predetermined harmonics of the 9,960 Hz reference phasesubcarrier such that the receiver vulnerability to particular levels ofthese false VOR bearing introducing distortions may be completelychecked out.

A functional block diagram of the VOR 9,960 Hz harmonic generator of thepresent invention is illustrated in FIG. 2. The 9,960 Hz subcarrierfrequency, modulated so as to contain the 30 Hz reference phase signalof a VOR transmission, is identified as input 11 to the harmonicgenerator. Input signal 11 is applied to a 9,960 Hz bandpass filter 18such that the .output 19 from the bandpass filter is a signal from whichsubcarrier harmonics generated by the VOR signal generator supplying theinput 11 are eliminated. Thus, the output 19 from filter 18 is a clean"9,960 Hz subcarrier reference devoid of harmonics. This is necessarysuch that the harmonic generator has absolute control over the 9,960l-lz subcarrier harmonics to be subsequently combined with this cleansignal 19.

The 9,960 Hz subcarrier input 11 as supplied by VOR signal generator isadditionally applied to a high gain limiter 20 which generates a 9,960Hz subcarrier square wave 21 rich in odd harmonics. The square waveoutput 21 from high gain limiter 20 is applied to a positive slopedifferentiator 22 (that is, is differentiated on one slope only) so thateven harmonic energy can be generated. The resulting pulses 23 from thepositive slope differentiator 22 are applied to a further high gainlimiter 24 and the output 25 from high gain limiter 24 is, as indicatedin FIG. 2, a nominal width pulse having a repetition rate equal to theinstantaneous frequency of the frequency modulated 9,960 Hz subcarriersignal. The pulses 25 at the output of high gain limiter 24 are now richin second, third, fourth, and higher harmonics of the 9,960 Hzsubcarrier.

The pulse energy, rich in harmonics, developed in high gain limiter 24,is applied to separate bandpass filters. A first bandpass filter 26receives the pulse train 25 and is tuned at the second harmonic of the9,960 Hz signal. A second bandpass filter 27 receives the pulse train 25and is tuned at the third harmonic of the 9,960 Hz signal. A thirdbandpass filter 28 receives the pulse train 25 and is tuned at thefourth harmonic of the 9,960 Hz signal. The output 29 from the secondharmonic bandpass filter 26 is additionally applied to a two pole, 30kHz roll-off low pass filter 32 to provide additional rejection offourth harmonic energy. The bandpass filters 26, 27, and 28 areindicated functionally as including respective level adjust controls 36,37, and 38. These gain controls incorporated with each of the bandpassfilters 26, 27, and 28 permit independent adjustment of the levels ofthe second, third, and fourth harmonic outputs from the filters.

The filtered 9,960 Hz subcarrier l9 (devoid of harmonics) and thecontrollable level second, third, and fourth harmonics of the 9,960 Hzsubcarrier (33, 30, and 31, respectively) together with the 30 Hzvariable phase signal 12 from the VOR signal genrator are applied to alinear mixer/amplifier 34 from which is developed the compositemodulation signal 14 containing the 30 Hz variable phase signal, the9,960 He reference phase subcarrier, and independently level controlledsecond, third, and fourth harmonics of the 9,960 Hz reference.

A functional schematic diagram of an embodiment of a harmonic generatorin accordance with the present invention is shown in FIG. 3. The 9,960Hz bandpass filter 18 is illustrated as being implemented with acommercially available type A709 functional element to provide thebandpass filtering function to which the 9,960 Hz subcarrier referenceinput is applied. High gain limiter 20 is implemented with a furthercommercially available type 1.1.A709 functional element from which thesquare wave signal 21 is developed. High gain limiter 20 is followed bya conventional positive slope differentiator network 22 with the output23 thereof being applied to the high gain limiter 24, also implementedfrom a commercially available type ;/.A709 functional element. The pulsetrain 25 from high gain limiter 24, rich in harmonics, is appliedthrough three controllable gain bandpass filters 26, 27, and 28, eachimplemented with a commercially available type A741 functional element,with independent level adjust effected by variable resistance members36, 37, and 38, respectively. The two pole, 30 kHz rolloff low-passfilter 32 is seen to be embodied as an RC filter network. The output 33from filter 32, the output 30 from the third harmonic bandpass filter27, and the output 31 from the fourth harmonic bandpass filter 28 arelinearly mixed and applied to an amplifier 34, the latter implementedwith a further commercially available type nA709 functional element.

The 30 Hz variable phase input signal 12, which would be supplied to theharmonic generator from an external source such as VOR signal generator10 of FIG. 1, is applied to a further level control network 35 to permitselected level application of the 30 Hz variable phase signal to theoutput amplifier 34. The output 14 from amplifier 34 comprises thecomposite VOR modulation signal including a 30 Hz variable phase signal,a 9,960 Hz reference phase input signal, and precisely selected levelsof the second, third, and fourth harmonics of the 9,960 Hz referencesignal.

The present invention thus: provides a harmonic generator from which issupplied controlled amounts of 9,960 Hz subcarrier harmonic distortionfor usage in testing the vulnerability of VOR receivers to thesedistortions.

Although the present invention has been described with respect to aparticular embodiment thereof, it is not to be so limited. The inventionhas been described in the environment of a 9,960 Hz harmonic generatorfor usage in VOR receiver testing since there exists a present andurgent need for such a test capability. It is to be realized that thepresent invention might equally be applicable to the generation of otherfrequency signals and selected level harmonics thereof for otherpurposes, thus changes might be made in the present invention asdescribed which fall within the scope of the present invention asdefined in the appended claims.

I claim:

1. Means for generating a composite signal comprising a fundamentalfrequency and selectively controllable levels of predetermined harmonicsof said fundamental frequency from a fundamental frequency generatingsource, comprising first frequency selective means receiving said sourcefrequency and producing a first output signal comprised of saidfundamental frequency devoid of harmonics, harmonic frequency generationmeans receiving said source frequency and developing a square waveoutput signal having a periodicy defined by said source signal and richin odd and even harmonics of said source signal, said harmonic frequencygenerating means comprising a first high gain limiter to which saidsource input signal is applied, a positive slope differentiator to whichthe output from said first high gain limiter is applied as input, and afurther high gain limiter to which the output from said positive slopedifferentiator is applied as input, a plurality of further frequencyselective devices to which the output from said further high gainlimiter is commonly applied, said plurality of further frequencyselective devices respectively passing frequency componentscorresponding to the second and successively higher harmonics of thesaid source frequency, and means for linearly mixing the outputs fromsaid first frequency selective device and each of said further frequencyselective devices the output from said means for mixing comprising anoutput signal comprised of said source signal fundamental frequency andthe predetermined harmonics thereof as developed by successive ones ofsaid further frequency selective devicesv 2. Means as defined in claim 1wherein each of said further frequency selective devices includes meansfor adjusting the signal output level therefrom.

3. Means as defined in claim 1 wherein each of said first and furtherfrequency selected devices comprises a bandpass filter.

4. Means as defined in claim 2 wherein each of said first and furtherfrequency selected devices comprises a bandpass filter.

5. Means as defined in claim 1 comprising a still further frequencyselected device receiving the output from that one of said furtherfrequency selected devices passing the second harmonic of said sourcefrequency fundamental, said still further frequency selective deviceproviding suppression of the second harmonic output of said secondharmonic frequency selective device, and the output from said stillfurther frequency selective device being applied to said mixing means.

6. Means as defined in claim 2 comprising a still further frequencyselected device receiving the output from that one of said furtherfrequency selected devices passing the second harmonic of said sourcefrequency fundamental, said still further frequency selective deviceproviding suppression of the second harmonic output of said secondharmonic frequency selective device, and the output from said stillfurther frequency selective device being applied to said mixing means.

1. Means for generating a composite signal comprising a fundamentalfrequency and selectively controllable levels of predetermined harmonicsof said fundamental frequency from a fundamental frequency generatingsource, comprising first frequency selective means receiving said sourcefrequency and producing a first output signal comprised of saidfundamental frequency devoid of harmonics, harmonic frequency generationmeans receiving said source frequency and developing a square waveoutput signal having a periodicy defined by said source signal and richin odd and even harmonics of said source signal, said harmonic frequencygenerating means comprising a first high gain limiter to which saidsource input signal is applied, a positive slope differentiator to whichthe output from said first high gain limiter is applied as input, and afurther high gain limiter to which the output from said positive slopedifferentiator is applied as input, a plurality of Further frequencyselective devices to which the output from said further high gainlimiter is commonly applied, said plurality of further frequencyselective devices respectively passing frequency componentscorresponding to the second and successively higher harmonics of thesaid source frequency, and means for linearly mixing the outputs fromsaid first frequency selective device and each of said further frequencyselective devices the output from said means for mixing comprising anoutput signal comprised of said source signal fundamental frequency andthe predetermined harmonics thereof as developed by successive ones ofsaid further frequency selective devices.
 2. Means as defined in claim 1wherein each of said further frequency selective devices includes meansfor adjusting the signal output level therefrom.
 3. Means as defined inclaim 1 wherein each of said first and further frequency selecteddevices comprises a bandpass filter.
 4. Means as defined in claim 2wherein each of said first and further frequency selected devicescomprises a bandpass filter.
 5. Means as defined in claim 1 comprising astill further frequency selected device receiving the output from thatone of said further frequency selected devices passing the secondharmonic of said source frequency fundamental, said still furtherfrequency selective device providing suppression of the second harmonicoutput of said second harmonic frequency selective device, and theoutput from said still further frequency selective device being appliedto said mixing means.
 6. Means as defined in claim 2 comprising a stillfurther frequency selected device receiving the output from that one ofsaid further frequency selected devices passing the second harmonic ofsaid source frequency fundamental, said still further frequencyselective device providing suppression of the second harmonic output ofsaid second harmonic frequency selective device, and the output fromsaid still further frequency selective device being applied to saidmixing means.